SW Japan Arc Research Articles

Miocene lamprophyre and felsic volcanics in the outer zone of SW Japan: arc building through multiple sources in an active convergent margin

ABSTRACT Subduction-related magmatism is a significant contributor to arc building and continental growth. In the active convergent margin of the SW Japan arc, voluminous volcanic rocks and mafic dykes formed through subduction and back-arc rifting during Miocene. In this study, we investigate a suite of felsic volcanic rocks (rhyolite and dacite) and lamprophyres from central Shikoku Island, which occur in the north-central part of the Chichibu belt and Sanbagawa belt in the Outer zone of the SW Japan arc. Whole-rock geochemistry results show that the rhyolite and dacite samples are peraluminous, belong to the high-K calc-alkaline series, show LREE and LILE enrichment, HREE and HFSE depletion that suggest arc volcanic affinity. The lamprophyre samples belong to alkali lamprophyre, show LREE and HFSE enrichments, and display OIB (oceanic island basalt)-like geochemical features. The 206Pb/238U weighted mean ages of the zircons from the rhyolite and dacite samples range from 14.39 ± 0.21 Ma to 13.92 ± 0.30 Ma, whereas the lamprophyre shows 206Pb/238U weighted mean age of 12.21 ± 0.35 Ma. Zircon Lu-Hf isotope data suggest that the rhyolite and dacite were derived from the remelting of the arc crustal rocks and that the lamprophyre was derived from depleted mantle sources and was contaminated by crustal materials. The felsic volcanic rocks (rhyolite and dacite) can be correlated to Miocene (15.5 Ma to 13.5 Ma) magmatism in the Outer Zone of the SW Japan arc. The formation of lamprophyre dykes in the SW Japan arc may be related to the subhorizontal mantle flows beneath the SW Japan arc associated with the opening of the Japan Sea back-arc basin or asthenosphere upwelling through the slab-tear of the Philippine Sea Plate beneath the SW Japan arc. Our study shows that the SW Japan Arc was built by multiple magmatism from diverse sources in an active convergent margin.

Petrogenesis of Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc: Temporal change of arc volcanism during the back-arc spreading in the Japan Sea

Petrogenesis of Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc: Temporal change of arc volcanism during the back-arc spreading in the Japan Sea

Neotectonics at the SE Continental Margin of the Korean Peninsula: Implications for the Back-Arc Region Behind the SW Japan Arc

Neotectonics at the SE Continental Margin of the Korean Peninsula: Implications for the Back-Arc Region Behind the SW Japan Arc

Correlation of Seismicity With Faults in the South Korea Plateau in the East Sea (Japan Sea) and Seismic Hazard Assessment

The South Korea Plateau (SKP) is a structural high at the eastern continental margin of the Korean Peninsula. Earthquakes occur frequently in the SKP although they are not larger than ML5.0 in magnitude. The SKP is a region of highly rifted continental crust engraved with rifts created during back-arc rifting in the NW Pacific subduction zone that led to the separation of the SW Japan Arc in the Cenozoic. The Bandal, Onnuri, and Okgye Rifts are well-defined rifts in the SKP. Multichannel seismic profiles show that the rifts in the SKP are bounded by large-offset normal faults and their subsided basement is deformed by domino-style faulting. A close spatial correlation is recognized between the epicentral locations of earthquakes with relatively large magnitude and the rift-bounding fault of the Onnuri Rift. The faults in the SKP is interpreted to be reactivated with strike-slip since the middle Miocene. The maximum expected magnitude of earthquakes (Mmax) computed statistically using the catalog including instrumentally recorded seismicity is ML5.42. However, Mmax estimated from the catalog including instrumentally recorded seismicity and historical records increases to ML6.67.

Sanbe volcano: Long‐term evolution of an arc magmatic system

AbstractA suite of samples was studied that represents the major explosive eruptions of Sanbe volcano, SW Japan. We demonstrate how rate of magma flux into the Trans‐Crustal Magmatic System (TCMS) presents a major control on the type and style of the subsequently forthcoming eruptions. Erupted products can be separated into two distinct groups. An older group is characterized by highly evolved, high‐K, LILE‐rich rhyolitic magmas, showing a supressed adakitic trace element signature (otherwise characteristic for the young stratovolcanoes in the SW Japan arc) with low Ca, Sr concentrations and a negative Eu anomaly. In contrast, the younger group (dominantly of andesitic—dacitic composition) displays a strong adakitic trace element signature with characteristic steep REE profiles and high Sr concentrations. An Eu anomaly is generally lacking here. The two groups are also distinct in their petrographic features, with the early group being almost aphyric showing simple log linear crystal size distributions and homogeneous, uniform mineral chemistries. In contrast, products of the younger group show complex crystal size distributions with diverse mineral compositions and abundant disequilibrium features. Our study shows that an initial high melt‐production rate allowed dehydration melting of lower crustal rocks leading to the formation of highly evolved K‐rich magmas. These magmas intruded into the shallow crust and produced two large Plinian rhyolitic, caldera forming eruptions. Subsequently the primary magma production rate decreased and the lower crust became too refractory for additional dehydration melting by these lower volume magma batches, causing the conventional adakitic magmatism to produced several additional eruptions of smaller magnitude, mainly of Sub‐Plinian or Pelean styles.

Outcrop-scale Deformation Structures of the Lower-Middle Miocene Strata in the Southernmost Part of the Akaishi Tectonic Zone, Central Japan: Their Significances for the Miocene Tectonic Rotation of the Eastern SW Japan Arc

Outcrop-scale Deformation Structures of the Lower-Middle Miocene Strata in the Southernmost Part of the Akaishi Tectonic Zone, Central Japan: Their Significances for the Miocene Tectonic Rotation of the Eastern SW Japan Arc

Contrasting volcano spacing along SW Japan arc caused by difference in age of subducting lithosphere

The SW Japan arc built by subduction of the Philippine Sea (PHS) plate exhibits uneven distribution of volcanoes: thirteen Quaternary composite volcanoes form in the western half of this arc, Kyushu Island, while only two in the eastern half, Chugoku district. Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by high-density heat flow data and a petrological model for dehydration reactions suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the non-volcanic forearc, to cause characteristic tectonic tremors and low-frequency earthquakes (LFEs) and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.

U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

AbstractWe present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, startedc. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Detection of microearthquakes and identification of their causative structures in the eastern offshore region of South Korea

A matched filter has been applied to continuous seismic records from the eastern offshore region of South Korea to identify small earthquakes undetected by routine processing. The resulting density of the regional earthquake catalog has increased by a factor of five relative to the detections reported by the Korea Meteorological Administration. High-precision locations of newly identified microearthquakes and previously located earthquakes cluster into streaks that delineate structures responsible for the seismicity associated with the Hupo Fault offshore of the southern Korean Peninsula. Microearthquakes in the northern and southern segments of the fault zone exhibit different behaviors: earthquakes in the northern segment occur at a regular temporal interval, while those in the southern segment occur in bursts. The identified fault system dips steeply to the west at shallow (<10 km) depths, with the dip angle becoming shallower at greater depths. A comparison of the microearthquake locations along the previously identified faults in the study area suggests reactivation of the Hupo Fault that formed as a listric normal fault via back-arc rifting associated with separation of the SW Japan Arc during the late Oligocene to early Miocene. The microseismicity data indicate that the fault has the largest fault plane identified to date using seismological techniques in onshore and offshore areas of the Korean Peninsula. Based on empirical relationships between earthquake magnitude and the potential rupture area, the fault system could generate a magnitude 6.5 earthquake if the entire fault plane outlined by the microseismicity ruptured in a single event.

Palaeoecological effects of back-arc spreading on Miocene benthic communities, SW Japan Arc

During the early to middle Miocene, rapid development of backarc basins, now known as the Sea of Japan, occurred in the NE margin of the Asian Continent. Then the entire SW Japan Arc (the arc) was rotated clockwise very rapidly. Such an event was associated with the opening of another backarc basin located to the south of the arc. Immediately after this opening, the very young, hot oceanic plate began to subduct underneath the arc. This subduction caused rapid uplifting and enhanced the denudation of the arc, in the post-early Miocene.The lower to middle Miocene Misaki Group is a 3,000 m-thick siliciclastic forearc basin fill, consisting of wave-dominated shallow-marine and overlying braid-plain deposits. Its defining features are the low ichnodiversity and paucity of ichnofabrics. For instance, up to 96% of the total thickness of the measured offshore deposits are non- or very slightly-bioturbated, despite the other “normal” offshore deposits are generally highly bioturbated. Although the less bioturbated sediments were previously considered as anoxic deposits, anoxic water was unlikely to develop within the highly open wave-dominated shallow-marine settings.The highly active tectonics associated with the spreading of the two backarc basins and the forced subduction most likely caused sediment mass-production in the hinterland, as evidenced by the sedimentological features of the coeval Kuma Group, and over-sedimentation (highly frequent and rapid sedimentation) that was taken place across the entire Misaki Basin. This over-sedimentation, supported by a series of evidence, such as the pervasive soft-sediment deformation across the group, estimated rate of net aggradation (more than 136 to 230 cm/kyr), etc., most likely induced the colonisation of depauperated fauna, and diluted their ichnofabrics, across the entire basin. The extremely active tectono-sedimentation played an important role in the development of the basin-wide anomalous benthic ecosystems and ichnofabrics during the early to middle Miocene.

Crystal uptake into aphyric arc melts: insights from two-pyroxene pseudo-decompression paths, plagioclase hygrometry, and measurement of hydrogen in olivines from mafic volcanics of SW Japan

Abstract Minerals of mafic rocks from the SW Japan arc have been studied to deduce P – T –X H2O conditions and their variations in mafic arc magmas. Two-pyroxene thermobarometry of magmas from several volcanoes yields constant temperatures and variable pressures. MELTS fractional crystallization modelling is employed to show that such ‘pseudo-decompression paths’ (PDPs) are artefacts that derive from uptake of pyroxene antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded mafic melts. It is shown that PDPs can be used to constrain oxygen fugacities and initial water contents of the intruded magmas. These constraints, and plagioclase hygrometry, indicate that initial melt H 2 O contents change systematically along the SW Japan arc. Direct determination of hydrogen in olivine by secondary ion mass spectrometry yields consistently low olivine H 2 O contents of 11±4 ppm (1σ), with little, if any, along-arc variations. MELTS modelling indicates that crystallization of calcic plagioclase and olivine dominantly occurs during upper crustal differentiation of mafic melts. The combined data indicate that aphyric melts are released from the mantle wedge, taking up most if not all crystals from previously intruded plutonic rocks during rapid magma ascent to the surface.

Origin of Late Oligocene to Middle Miocene Adakitic Andesites, High Magnesian Andesites and Basalts from the Back-arc Margin of the SW and NE Japan Arcs

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas.

Geochemical mapping in Shikoku, southwest Japan

Abstract Geochemical mapping of Shikoku in the SW Japan Arc was conducted using stream sediments (

領家帯に記録された東アジア東縁の白亜紀イベント

In the Ryoke belt formed at the eastern margin of East Asia during mid-Creataceous time, the most important geological record is that of felsic igneous activity, which may have been caused by the subduction of a young (hot) oceanic lithosphere. The mantle-derived arc magmas formed due to subduction of the hot lithosphere underplated the lower crust. Continuous underplating of the mantle-derived magmas resulted in partial melting of the lower crustal mafic to intermediate rocks to produce granite magmas. The granite magmas rose and were emplaced at a mid-crustal depth. They caused thermal perturbation around rocks of the Jurassic accretionary complex (Mino–Tamba belt) and formed the resultant low-pressure/high-temperature regional contact metamorphism (Ryoke metamorphism). At the time of Ryoke metamorphism, during subduction of a hot oceanic lithosphere with high obliquity, a low-angle detachment fault formed at the forearc region of the mid-Cretaceous SW Japan arc. Following this event, subduction of an inactive oceanic ridge or a very hot oceanic lithosphere with low obliquity occurred; a compressional stress regime occurred at the forearc region, forming granite mylonites and large-scale upright folds associated with crustal displacement along the detachment fault. After this event, the angle of subduction decreased gradually; the location of the felsic igneous activity moved toward the continent.

Erratum to: Seismicity and crustal structure in the vicinity of the southern Itoigawa-Shizuoka Tectonic Line

The southern part of the Itoigawa-Shizuoka Tectonic Line represents the boundary between the rapidly uplifting Akaishi range and the Kofu basin. The tectonic relation between the different parts of the fault system and the surrounding geological units is yet to be fully explained. In order to reveal seismic activity that might be related to the southern segments of the fault system, a seismic array observation was conducted in the autumn of 2005. The arrival times of 434 local earthquakes, two vibroseis shots and eleven explosive shots were used in a joint inversion to determine earthquake locations and a 3-D velocity structure of the crust. The relocated events are in good correlation with the estimated deeper extension of the active faults in the area. The seismic velocity model jointly obtained coincides with the geological structure in the area of study. In particular, a thick moderately low Vp zone was found beneath the Akaishi range and a high velocity zone beneath the Kofu basin. The moderately low velocity was attributed to the relatively young sedimentary rocks that form the accretionary prism units of the SW Japan arc and the high velocity to the older igneous rocks constituting the Izu-Bonin arc crust.

Seismicity and crustal structure in the vicinity of the southern Itoigawa-Shizuoka Tectonic Line

The southern part of the Itoigawa-Shizuoka Tectonic Line represents the boundary between the rapidly uplifting Akaishi range and the Kofu basin. The tectonic relation between the different parts of the fault system and the surrounding geological units is yet to be fully explained. In order to reveal seismic activity that might be related to the southern segments of the fault system, a seismic array observation was conducted in the autumn of 2005. The arrival times of 434 local earthquakes, two vibroseis shots and eleven explosive shots were used in a joint inversion to determine earthquake locations and a 3-D velocity structure of the crust. The relocated events are in good correlation with the estimated deeper extension of the active faults in the area. The seismic velocity model jointly obtained coincides with the geological structure in the area of study. In particular, a thick moderately low Vp zone was found beneath the Akaishi range and a high velocity zone beneath the Kofu basin. The moderately low velocity was attributed to the relatively young sedimentary rocks that form the accretionary prism units of the SW Japan arc and the high velocity to the older igneous rocks constituting the Izu-Bonin arc crust.

Mid-crustal horizontal shear zone in the forearc region of the mid-Cretaceous SW Japan arc, inferred from strain analysis of rocks within the Ryoke metamorphic belt

The strain geometry of a Cretaceous low- P/high- T metamorphosed accretionary complex in SW Japan is one of the key geological constraints in understanding the Cretaceous tectonics of the forearc region at the eastern margin of Eurasia. We studied the strain geometry of upper greenschist to lower amphibolite facies metamorphic rocks from the mid-Cretaceous low- P/high- T Ryoke metamorphic belt, which during the mid-Cretaceous were located at mid-crustal depths close to the volcanic front. Strain analysis focused on deformed radiolarian fossils in metachert and sandstone/chert clasts within metapelite. The maximum stretching direction ( X-direction) of the strain ellipsoid of the schistosity-forming deformation at the peak metamorphism is oriented E-W, and the XY-plane of the strain ellipsoid developed parallel to schistosity. The metachert and metapelite exhibit plane strain to general flattening strain that probably largely resulted from the schistosity-forming deformation. Based on the results of the previous strain analyses and this study, it reveals that the schistosity-forming deformation occurred under bedding-normal shortening, resulting in general flattening strain throughout the Ryoke metamorphic belt. The X-direction of the strain ellipsoid of the schistosity-forming deformation is oriented parallel to the length of the Ryoke metamorphic belt throughout the SW Japan arc. The schistosity formed parallel to bedding planes that developed horizontally in non-metamorphosed accretionary complexes of SW Japan, indicating horizontal XY-plane of the strain ellipsoid. These observations indicate the occurrence of horizontal shear deformation within the middle crust (∼15 km depth) at the time of the low- P/high- T metamorphism. The deep-seated horizontal shear zone and the shallower incipient Median Tectonic Line (MTL) fault system could result in a crustal-scale detachment fault in the forearc region of the mid-Cretaceous SW Japan arc. After the formation of the detachment, the left-lateral pull-apart forearc basins, Onogawa-Izumi Group distributed on the north side of the MTL and the southern marginal mylonite zone might be formed by the sinistral movement along the detachment fault.

Comparative morphotectonics in the Himalayan foreland and the forearc of Southwest Japan

The Southwest Japan, which is developed along a subduction boundary, is not a typical island arc. It has rich granitoid and crystalline, poor active volcano, thick upper crust and so on. These geophysical characteristics are in common with a collision mountain range such as the Himalayas. Geodetic surveying and seismology in SW Japan are ahead with those in the Himalayas. On the other hand, the Himalayan front that is a subaerial plate boundary has advantage for geologic and geomorphic studies. Therefore, comparative studies in the two boundaries aid to clarify their active tectonics by recovering the deficiency of each other. Reflecting crustal thickness and shortening rate, the Himalayas is about twice larger in relief energy and far shorter in topographic wavelength than the SW Japan arc. In a seismotectonic sense of the boundary fault, the arc is divided into proto-thrust zone, imbricate thrust zone, multi-decollement zone, and earthquake thrust zone (Kagami et al. 1983). Morphostructural similarity in the Himalayas and the SW Japan is obvious as close to the plate boundaries. The proto-thrust zone is just on the plate boundary and is filling up with thick sediments. These characteristics are in common with the Ganges foredeep. Geostructure and morphology on the imbricate thrust zone is similar to those of the Sub-Himalaya. Both areas are deformation fronts with relatively low relief, and consist of Neogene-Quarternary deposits, which are sliced by imbricate thrusts in piggyback sequence. In a sense of lithostrigraphy, the area above the multi-decollement zone can not correlate to the Lesser Himalaya directly. However, nappe structure and its related crustal processes in the Lesser Himalaya resemble the development of the multi-decollement. In addition, its geomorphic pattern composed of the Mahabharat Range and the midland depression also corresponds to a series of outer ridges (Tosa-bae etc.) and forearc basins (Muroto trough etc.) on the decollement zone. The area above the earthquake thrust zone forms steep slope from the Muroto Trough to the Shikoku Island. Relief energy of the slope is the largest in the SW Japan arc. Though there is rare similarity in lithostratigraphy, south face of the Higher Himalaya may equivalent to the slope on the earthquake thrust zone by its remarkable topographic gap. The above mentioned meso-scale morphotectonic features, which are over 10 km in topographic wavelength, are essentially formed by stable slip of flat-lying boundary faults. A dislocation model (Kimura and Komatsubara 2000) can reproduce it. In this model, ridges and basins and of the Himalayan foreland are mainly simulated by slip on thrust ramps branched off from the decollement. Also, slip on the decollement may be considered. Its topographic profile is in accord with the interseismic (post-seismic) crustal movement. On the other hand, remarkable uplift by the 1905 Kangra earthquake, which is considered to be a huge inter-plate earthquake, was observed along topographic depressions in the sub-Himalaya (Yeats and Lillie 1991). This is simulated chiefly by rupture on the decollement beneath the Lesser to Higher Himalayas. These facts show that coseismic crustal movement is less effective to form morphotectonic sequence of the Himalayas. Similar trend is known in the SW Japan Arc. For example, seismic uplift by the 1946 Nankai earthquake (M 8.0) was only observed on the south margin of the Shikoku Island (Ohmori 1978) that is topographic depression in front of the pre-Neogene prism. Regardless, seismic slip by inter-plate earthquake occurs beneath a frontal margin of a pre-Neogene accretionary wedge, which forms as a steep topographic gap. It means that implications of the next inter-plate earthquake are hidden in the Lesser Himalaya to the southern slope of the Higher Himalaya. Intensive research of seismology and geodesy should be focused to these zones. In addition, the areas above the earthquake thrust zone and their backarc side, there are minor active faults, which are causes of intra-plate earthquakes. Though the earthquakes are relatively small and their recurrent interval are usually over 103 years, they must be care as secondary important seismic hazards. Their behavior can be clarified by geomorphic and trenching survey.

P58 九州南部に産する火山岩起源物質中のホウ素含有量の空間的・時間的変化(ポスターセッション,日本火山学会2006年秋季大会)

P58 九州南部に産する火山岩起源物質中のホウ素含有量の空間的・時間的変化(ポスターセッション,日本火山学会2006年秋季大会)

Os, Nd, and Sr isotopic and chemical compositions of ultramafic xenoliths from Kurose, SW Japan: Implications for contribution of slab-derived material to wedge mantle

We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan. The Kurose xenoliths have less radiogenic Os isotopic ratios ( 187Os/ 188Os = 0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.